1. Field of the Invention
The present invention relates to a chromatograph, and more particularly to a chromatograph which utilizes a moving average method to reduce noise in data.
2. Description of the Related Art
For noise reduction, there are the following three methods which are generally used in chromatography:
A first method employs a RC circuit composed of a coil and a capacitor in an analog signal output stage to reduce short-cycle noise.
A second method converts an analog signal to a digital signal which is then subjected to exponential smoothing, having a similar effect to the RC circuit, to derive an output.
A third method, as shown in JP-A-62-291562, converts an analog signal to a digital signal, and then applies a moving average method using a plurality of data points of the digital signal for noise reduction, after which an output is derived. For an equation used in the moving average, a Savostzky & Golay's equation or the like is suitable.
When the first and second techniques employ a circuit or a smoothing equation providing a large noise reduction effect, distortion is also introduced into necessary signals, thereby causing a degradation in signal separation. In addition, such a circuit or a smoothing equation also reduces a signal height (peak height), so that an improved signal-to-noise ratio (hereinafter referred to as the "S/N ratio"), which is the essential object of the techniques, cannot be achieved.
The third technique samples an analog signal at regular intervals to convert it to a digital signal, wherein digital samples at a predetermined number of sampling points before and after each sampling point are averaged at every sampling time. This technique can reduce noise without introducing distortion in necessary signals and accordingly improve the S/N ratio by appropriately increasing the number of data samples used in the moving average or increasing the data interval (sampling interval).
However, the third method suffers from a delay in an output signal with respect to a true signal. For example, when the moving average is applied using 11 data samples at sampling points spaced by a sampling interval of 100 ms, an output signal is delayed by 500 ms from a true signal (later described). Particularly, as the number of data points (the number of samples) used in the moving average is increased to have a larger noise reduction effect, the delay also becomes larger. Also, when the noise reduction effect is frequently changed, the delay time similarly changes frequently from the true signal.
The chromatograph uses a holding time for identifying components of a sample. The holding time extends from the time a sample is injected into a column and components of the sample are eluded from the column to the time the components are observed as peaks. Therefore, a delay in providing output data, and particularly a varying delay due to a change in the measuring characteristics would hinder accurate identification.